Force limiting device



Aug. 15, 1961 A. E. FARR ET AL FORCE LIMITING DEVICE Filed May 27, 1960 United States Patent-O 2,996,270 FORCE LIMITING DEVICE Alton E. Farr, Rolling Hills, and Edward 'F. Hiltz, In, Los Angeles, Calif., assignors to Douglas Aircraft, Inc Santa Monica, Calif.

Filed May 27, 1960, Ser. No. 32,316 6 Claims. (Cl. 244-76) This invention pertains to force limiting devices. and more particularly to a device which will limit the amount of force imparted by a servo mechanism or servo motor to certain control surfaces on an airplane or to their control means.

One of the principal purposes of the device of this invention is to provide a means for limiting to a safe value the control force resulting from the torque output developed during certain flight and emergency conditions by an aircraft autopilot elevator control servo motor-in order to prevent the transmission of excessive forces to the elevator control cables and correspondingly overstressing the control surfaces themselves. By the same means excessive aerodynamic control effects are avoided.

The deviceis so designed and adjusted that during ordinary operation of the airplane the forces imparted to the control cables by the servo motor will not exceed the normal pre-load value of the device. If the servo motor receives a signal for maximum action or if it should malfunction, then it may well apply a force to the control cables which exceeds their design load and likewise overstress the elevator surface as well as the airplane itself by causing a violent maneuver. The novel mechanism of this invention provides for yieldable bodily movement by the servo motor in immediate response to generation of forces beyond the design limit and thus prevents their transmission to the aerodynamic control system.

The force necessary to displace the elevator to any given degree is a function of the speed of the airplane.

Thus, at high speeds the control forces must be greater than at low speeds for the same elevator movement. However, the aerodynamic elfect of the elevator increases rapidly with speed and hence only relatively small movements are needed at high speeds to produce the maximum desired effect. The net result is that less force is required or desirable at high speeds than at low speeds. Therefore the yielding force control mechanism must provide less resistance to yielding at high speeds than at low speeds. The problem of the necessary adjustment is solved in this invention by taking advantage of the fact that flaps are always retracted at .high speeds and extended only at low speeds, as for landing. The position of the flaps is used to control the resistance of the device. As the flaps are lowered to extended position the force control mechanism is adjustedto increase its resistance to yielding movement of the servo motor.

Briefly described, the force control or force limiting mechanism of this invention includes a support member fixed on the structure of the airplane and a servo motor pivotally mounted on the support member. The servo motor has a driving drum from which extend two runs of drive cables which are connected to the control cables leading to the elevator or other aerodynamic control surface. A lever is pivotally mounted on the support member and one end is connected to the servo motor by a coil spring. The other end is attached to cables which extend to the flap operating region of the airplane and are operated by movements of the flap to swing the lever about its pivot. In the flap-up position the spring is tensioned to a relatively small extent. As the fiap is lowered, the lever is swung to a position increasing the tension in the spring. The spring tension controls the yieldability of the servo motor.

The entire mechanism is so arranged and adjusted that,

2 when the flaps are fully down and the resistance is great est, normal-operation of the servo motor will cause full operation of the control cables with little or no yielding but abnormal high torque action of the servo motor will cause sufficient yielding to limit the control force to predetermined safe limits. When the flaps are retracted, the resistance is less and there will be correspondingly more yielding of the servo motor about its pivot.

These and other features and advantages of the invention will become more apparent as the description proceeds in connection with the attached drawing, in which:

FIG. 1 is a broken perspective view of an airplane in which certain control mechanisms including the force limiting mechanism are shown largely in diagrammatic form; and

FIG. 2 is a side elevational view, partly in section, showing the form and arrangement of the essential elements of the mechanism.

Referring first to FIG. 1, an airplane 10 includes a nose portion 12, a mid portion 14, and a tail portion 16. The nose portion, as is usual, contains a pilots compartment 18 including a control column 20. From the column support 22 extend a pair of elevator control cables 24, 26. These may extend directly to the elevator but in most modern large high speed aircraft they lead to a servo motor, not shown. From such a servo motor, control cables 28, 30 extend to elevator actuating lever 32 which rotates elevator 34 about its pivotal supports.

A support member 36 is fixed to the structure of the tail portion of the airplane and a bracket as support mem ber 38 is rigidly attached thereto. Referring to both figures, it will be seen that an arm 40 is pivotally attached to support 38 at 42. A servo motor 44 is mounted on arm 40 by conventional fastening means and the assembly is free to swing clockwise or counterclockwise as viewed in FIG. 2. The servo motor carries a drum 46 on which are wound several turns of cable, terminating in drive cables 48, 50 which are joined to control cables 28, 30 at 52, 54. Electrical conductor means 56 extends to the autopilot 58 in the pilots compartment and the servo motor is controlled by signals therefrom.

A lever 60 is pivotally mounted intermediate its ends on bracket 38 at 62. A first end 64 serves as a connection for the lower end of coil spring 66. Arm 40 is provided with a boss 68 to which the upper end of the spring is attached. In the position shown, the spring is under that amount of tension which will produce the proper resistance to yielding of the servo motor to loads produced in the drive cables in the higher speed regimes. When lever 60 is rotated counterclockwise to a predetermined degree it will increase the tension in the spring and hence resistance to yielding of the servo motor to the extent necessary for the lower speed regimes. In addition, the

resistance to yielding of the servo motor is incurred by the increase of the lever arm between the pivot point 42 and the line of force of the spring 66 This provides an added variable to increase the resistance to yielding of the servo motor.

The appropriate movement of lever 60 is accomplished by its operative connection to flap 70 movably mounted on wing 72. The second end 74 of the lever is connected to cables 76, 78 which extend to and are connected to lever 80 carried by shaft 82. The shaft is rotated clockwise, as seen in FIG. 1, when the flap is lowered and counter-clockwise when it is retracted.

To keep the mechanism within its designed operable limits the drum 46 has attached to it a stop 55 which engages a fixed stop 57 on the arm 40 at the desired outer limits of rotation of the drum 46.

When the mechanism is at its low output position or seen in FIG. 2, the point of attachment of spring 66 to boss 68, the first end 64 of lever 60, and the pivot point d2 of lever 69 define a straight line; orientation of these points provides a fail safe feature. If one of the cables 76, 78 should break, the lever 60 will pivot about 452 un il the poin s of at achmen of he spring are in with it and t lo o tput posi on has been each d- It wi l n be s en a ser o mo or 4& is b ed to exe t a en on orc i drive cables 45, 5G a all th amo n o r istance to yi lding being contro led by flap pos t on o th t r sistance is higher a low speeds to accomplish he necessa y re e hrow of the ele ator. At the me time if maxim m to que is appli d to as in th i ction of rot t on, th exce sive torce in cab 4: o 5 ll pull h s r o motor a ains the re c o sp in 66 and thus reduce the loads i g in the l a l s and el v tor o a p det n s e l As he m 40 otates, he in: cre sed si n in h p in 66 s ofiset by a de rease oi the lever arm between the pivot point 42 and the line of force o th pr 66 hich produ es a fiat spring ra that is, a constant f e n the dri e ables 50, While the elevator is used as an example, it is obvious that the forc m t g h n sm. can e ppli d to any cont surface where the problem arises.

I l b pp r to hose kill d n th art that vari= ous ch n an m d fic tions can. be made in the details of ons uc ion as ho n nd d scribed, and it te ded that all uch changes nd mo ific io s shall e embraced with e scope of h follo i cl ims- We claim:

1. In an airpl n in l ding a p n o ah y mounted on a wing thereof and a pivotally supported elevator, for controlling the movement of said elevator, eorn g: a pair of control cables operatively attached to sa d lee. vator; a servo motor; drive cables extending from said servo motor and attached to said control cables; a support member fixedly mounted in said airplane; means mount; ing said servo motor for rotation thereon; a lever pivotal y mounted intermediate its ends on said support, spring means extending between said servo motor and a first end of said lever to yieldingly rotate said servo motor to produce tension in said drive cables; and means opera: tively connecting the second end of said lever to said flap to produce rotation of said lever in accordance with the position of said flap and thereby vary the resistance at said spring means and the tension in said drive cables.

2. In an airplane including a flap movably mounted on a wing thereof and a pivotally supported elevator, means for controlling the movement of said elevator, eomprising: a pair of control cables operatively attached'to said elevator; a servo motor; drive cables extending from said servo motor and attached to said control cables; a support member fixedly mounted in said airplane; means mounts ing said servo motor for rotation thereon; force control means movably mounted on said support member; spring means extending between said servo motor and said force control means to yieldingly rotate said servo motor to clockwise escalate produce tension in said drive cables; and means operatheir epnneetins said terse control rneans to sai flap to produce positioning of said force control means in accordance with the position of said flap and thereby vary the resistance of said spring means and the tension in said drive cables.

3. In an airplane including a flap movably mounted on a wing thereof and a pivotally supported elevator, means for controlling the movement of said elevator, comprising; a pair of eontrol cables operatively attached to saidelevator; a servo motor; drive cables extending from said servo motor and attached to said control cables; a suppor m mber fix dly mou ted in said a rp an means seen us said ser o t r for rotati n th r on; yieldable for pplying means c ried by said ppor and attach d to said servo mo to rotate the latter and produce tension in said drive cables; and means operatively connectapplyi g to said flap to vary the resistance thereot in a ordan e i h e p iti n of said flap and hereby ary the tension in said driv cabl s.

4, In an airplane incl din a flap a ly unted on a. ine the r 1? an a pivota y supp r e l va or, in ans tor controlling the mo emen of sai elevat r, eenrp ising= a pair et on r l cabl op ratively attached to said ele ator; a se, o mo or; d i e cables extend g train said ser o motor attached to Said control cabl s; a support member tixdly mounted in s i pl means mounting said ser o motor for rotation thereon;

yieldable force app ying me n att he to i s r motor to rotat t latter an p duce tension in i dri e cahles nd adjustable to vary id tension; and m ans ope able to adjust s'aidforee pp yin means to produce reduced tension in hpeed regimes and in r ased tension in low speed regimes- 5- Mean to control he movement of a p o n ro cables for an aerodynamie outrol surface f an airplane. comprising: a support; a servo motor pi otally mounted thereon; dri e cables extending tront aid servo motor for onnection to said. control c bles; sp in means onnes ed to said servo mo or to ot e the lat er and produ ension n aid dri e cables; adjustm nt means on said s pport and attaehed o said spring means to vary the r sistance thereof; and m ns pera le in accor a th the speed. regime of the airplane to ary the adjustm n means- 6- M ans o con rol he mo ement 05 a p f conrol e bles .for an aero ynamic control rface of n P an comprising: a supp a servo motor movably mounted n s id. pport; drive cabl s ext n ing f m s servo motor for connection to said control cables; adjusta le m ans yi ld n y urging said servo m t r n a dir ction to produce tension in said drive cables; and means operable in accordance; with the Speed regime of said air- No references cited. 

